Lady Bird Johnson, the former first lady who was married to President Lyndon B. Johnson, died Wednesday, according to a family spokeswoman. She was 94.

She died at her Austin, Texas, home of natural causes at 4:18 p.m. surrounded by family and friends, says the spokeswoman, Elizabeth Christian.

Just last month, she returned home after a week at Seton Medical Center, where she'd been admitted for a low-grade fever, the Associated Press reports.

As a girl she was, her nanny once said, "as pretty as a lady bird." That southern endearment stuck – much to the chagrin of a girl born Claudia Alta Taylor who one day became the wife of the 36th President.

Lady Bird Johnson had a lifelong love of nature, fueled by childhood memories of her mother walking in the woods barefoot, carrying a bouquet of flowers. After her mother died, when Johnson was just 5, the forests of her east Texas home became her balm.

"Nature mothered her," biographer Jan Jarboe Russell tells PEOPLE. "It was deeply emotional for her."

In 1934, three months after being introduced to him by a friend, she married Lyndon Baines Johnson, an up-and-coming aide to a Texas congressman, sealing the deal with a $2.50 ring from Sears, Roebuck.

Thrust into the role of First Lady after witnessing President Kennedy's assassination, she never tried to compete with her glamorous predecessor Jacqueline Kennedy.

Lady Bird said her attraction to LBJ was like that of "a moth to a flame."

She was devoted to her husband and worked tirelessly on issues that mattered to her: education (she was an early champion of Operation Head Start); civil rights (she campaigned for the Civil Rights Act of 1964 in the South when death threats kept her husband off the road); and nature, working to preserve native plants and limit highway billboards, even after LBJ's death in 1973.

"My special cause," she once said, "is to help pass on to generations the quiet joys and satisfactions I have known since my childhood."

Lady Bird and Lyndon Johnson had two daughters, Lynda Bird Robb, 63, and Luci Baines Johnson, 60.